The prior art is illustrated in European patent EP-A-0 415 822, U.S. Pat. No. 5,156,736 and French patent FR-A-2 743 002 which describes the most effective implementation.
A simulated moving bed comprises at least three chromatographic zones, advantageously four or five, each of the zones being constituted by at least one column or column section. At least one point between two zones acts to inject a feed to be fractionated and at least one point between two zones acts to inject an eluent or desorbent or solvent. Further, the simulated moving bed comprises at least one point for withdrawing an extract between the eluent injection point and the feed injection point located downstream in the desorbent circulation direction and at least one point for withdrawing a raffinate between each mixture injection point and the eluent injection point located downstream in the direction of desorbent circulation.
The set of columns or sections forms at least one closed loop containing at least one recycling pump, between two sections, which pump is flow rate regulated (between the first and last section, for example).
In general, the injection and withdrawal points are periodically shifted by at least one section in the same direction (upstream or downstream).
It is essential to observe the flow rates of the effluents which circulate from one zone to another and which must remain substantially constant in a given zone. A small variation in flow rate, even in a single zone, can have a very large influence on the separation results.
As an example, consider a counter current simulated moving bed comprising four zones with a recycling pump, two incoming streams, the desorbent and the feed, and two outgoing streams, the extract and raffinate.
Zone 1 is located between the desorbent and the extract; zone 2 is located between the extract and the feed; zone 3 is located between the feed and the raffinate; and zone 4 is located between the raffinate and the desorbent. The flow rates in the different zones are as follows when the pump is in zone 1, for example:
Flow rate in zone 1: pump flow rate; PA0 Flow rate in zone 2: flow rate in zone 1-flow rate of extract; PA0 Flow rate in zone 3: flow rate in zone 2+feed flow rate; PA0 Flow rate in zone 4: flow rate in zone 3-raffinate flow rate. PA0 1) Each time raffinate withdrawal is permutated from upstream to downstream of the recirculation pumps, despite pre-positioning of the raffinate control valve and anticipation of its operation so as to take into account the manoeuvring time of several seconds and the pressure difference of several bars, it can be seen that the pressure regulated at the bottom of one of the two adsorbers and the raffinate flow rate undergo a perturbation. PA0 2) Each time another of the principal streams is permutated from the head to the bottom of the adsorber, a minimal residual perturbation can be seen in the flow rate of the stream being permutated, and a perturbation is induced in the raffinate flow rate. PA0 1) Since the distillation column located downstream of the raffinate control valve is supplied at a pulsed flow rate, it has to be overdimensioned compared with a far more regular supply flow rate. PA0 2) A pulsed raffinate flow rate can cause pulsation in the internal flow rates, particularly in zones 3 and 4 of the simulated moving bed. So as to guarantee that the constituents of the raffinate cannot traverse zone 4, the flow rate in zone 4 has to be reduced, i.e., the desorbent flow rate has to be increased with respect to ideal operation. In order not to reduce the flow rate in zone 3 (and thus the feed flow rate and, in the end, the productivity of the unit), a loss of desired constituent in the extract has to be tolerated which is a little higher than that which would be obtained with ideal operation.
Any errors in the flow rates of the incoming or outgoing streams thus reflect on the recycling flow rate and thus must be controlled with precision.
Each time that one of the incoming or outgoing streams of the loop passes from one to the other side of the recycling pump, for example from a position immediately anterior to a position immediately posterior of the recycling pump when operating in simulated counter-current, two difficulties occur from the point of view of the regularity of the flow rates:
the first difficulty concerns the recycling pump, when it changes zone. It is very important that its flow rate is modified almost instantaneously and that the new flow rate, that of the new zone in which it is to be found, is precisely and stably regulated without the transition from one flow rate to the other being too slow (in the case of damped regulation) or with fluctuations about the new value (in the case of fast response regulation). PA1 The second difficulty concerns the flow rate of the stream entering or leaving the unit. The flow rate of that flux must be kept constant, and with very good precision, when its point of injection or withdrawal passes from a low pressure, that of the pump intake, to a high pressure, that of the pump discharge (the pressure difference corresponds to the pressure drop in the group of columns or column sections). PA1 in the case of the recycling pump, to change the flow rate from one zone to another; PA1 in the case of an effluent, to maintain the flow rate in the presence of a large variation in pressure conditions.
This first technical problem is related to changing the zones. It is actually very important that the flow rate passes instantaneously from a first value to a second desired value. As an example, it has been shown that a variation of 0.6% in the recycle flow rate produces a variation of 4.2% in purity. It has been shown that a flow rate regulator which stably regulates the flow rate to which a new set rate is supplied takes a certain time to regulate to the new value. As the transition from one flow rate to another must be very rapid, the gain of the regulator has to be high. In this case, regulation is not very stable. Thus we have to make a choice between rapid but fluctuating regulation or fine and stable regulation but with inertia. Neither of those two solutions is acceptable when regulating a simulated moving bed, which aims to produce high purities.
It is very important that these problems are overcome to obtain good separation results.
The solution proposed in the prior art consists of not allowing the regulator to act alone but, by using a computer or any other means which can act on the regulator, to cause that regulator to temporarily stop regulating, simultaneously to cause it to modify its action (pre-positioning the action) such that the new action imposed, such as a percentage valve opening, a current frequency for a motor, a voltage, etc . . . corresponds to the new conditions enabling the flow rate under consideration to be properly regulated and then, but in fact almost immediately, to restart the action of the regulator:
Thus good flow rates are obtained without oscillations and almost instantaneously. The ensemble of operations described take between 1/100 and 10 seconds and usually between 1/10 and 5 seconds, depending on the case.
The examples described in the text of the prior art are applicable to a pilot unit comprising 24 beds, for example, and to a single circulating pump.
When the process for regulating the flow rates of the prior art (FR 95/15526) is applied to the unit described in FIGS. 1A, 1B and 2, it can be seen that residual perturbations subsist, in particular in the raffinate flow rate (see FIGS. 3A, 3B, 3D, 3C).
An examination of these figures enables the nature of the problem to be better understood. FIGS. 1A and 1B show how the pressures and flow rates in a closed loop of 24 beds constituting the simulated moving bed are regulated. Two adsorbers 1 and 2 each comprise 12 beds 1.3 m in height and 7.6 m in diameter. Two pumps 9 and 10 circulate liquid inside the adsorbers. A flow meter 3 and a flow rate regulation valve 4 control a flow rate of between 1100 and 3200 m.sup.3 /h between adsorbers 1 and 2 with a very high precision (0.2%). A pressure controlled raffinate withdrawal valve 6 maintains a set pressure 5 at the intake of pump 9. A control valve 8 maintains a set pressure 7 at the inlet to pump 10. With reference to a cycle of 24 periods where the desorbent is injected into the first bed of adsorber 1, control valve 6 is in direct communication with adsorber 2 during periods 1 to 3 and 16 to 24 (FIG. 1b) and with adsorber 1 during periods 4 to 15. Between periods 3 and 4 raffinate withdrawal passes from the bottom of adsorber 2 to the head of adsorber 1; between periods 15 and 16, raffinate withdrawal passes from the bottom of adsorber 1 to the head of adsorber 2. On-off valves enable a given bed to be placed in communication with the withdrawal circuit, taking about 2.5 seconds to pass from the open position to the closed position (or the reverse). The average pressure drop in each of the two adsorbers is 4.2 bars. When control valve 6 is connected to the bottom of one of the two adsorbers, it is about 67% open; when it is connected to the head of one of the adsorbers, it is about 55% open, with the time taken to pass from 67% open to 55% open being about 2.5 seconds. In the prior art, the best results are obtained by causing control valve 6 to go to 55% open about 2 seconds before manoeuvring the on-off valves and by maintaining this degree of opening for about 6 seconds; at the end of the 6 seconds, the valve is re-set to automatic by the controller. It should also be noted that the best results are obtained with slightly different anticipation and maintenance times during two transitions from the bottom to the head of the adsorbers. Control valves 4 and 8 are manoeuvred using the same principle, however with different anticipation or retardation and maintenance times.
FIG. 2 shows the arrangement of the withdrawals and injections in the loop. The raffinate withdrawal circuit comprises 24 on-off valves numbered 601 to 612 and 651 to 662 (only valves 601 to 603 and 651 to 653 are shown); these 24 valves are connected to a pressure control valve via lines 60, 61 and 62. The extract withdrawal circuit comprises 24 on-off valves numbered 1201 to 1212 and 1251 to 1262 (only valves 1201 and 1253 are shown), these 24 valves are connected to a flow meter 11 and to a flow rate control valve 12 via lines 120, 121 and 122. The feed injection circuit comprises 24 on-off valves numbered 1301 to 1312 and 1351 to 1362 (only valve 1353 is shown); these 24 valves are connected to a pump 15, a flow meter 14 and to a flow rate control valve 13 via lines 130, 131 and 132. The desorbent injection circuit comprises 24 on-off valves numbered 1601 to 1612 and 1651 to 1662 (only valve 1653 is shown); these 24 valves are connected to a pump 18, to a flow meter 17 and to a flow rate control valve 16 via lines 130, 131 and 132.
FIGS. 3A, 3B, 3C and 3D show recordings of the four principal flow rates during a complete cycle. The set values for the flow rates for the desorbent (FIG. 3A), extract (FIG. 3C), and feed (FIG. 3B) are respectively 830 m.sup.3 /h, 320 m.sup.3 /h and 480 m.sup.3 /h. It can be seen that the amplitudes of the variation about the set value are plus or minus 5 m.sup.3 /h for the extract and feed and plus or minus 5 m.sup.3 /h for the desorbent. The average resultant raffinate flow rate (FIG. 3D) is 990 m.sup.3 /h. for this particular stream, the variations in amplitude are +/-50 m.sup.3 /h instead of the 20 m.sup.3 /h which should be expected (sum of the amplitudes of the variation in the other flow rates). Under these conditions, a para-xylene production of 74 t/h was produced, with a purity of 99.87% and a yield of 94.5%.
There are two types of perturbations:
The disadvantages of such perturbations in the raffinate flow rate are:
It is thus clear that the prior art cannot produce a highly regular raffinate flow rate, and for the other streams, each change in step gives rise to a small perturbation which is clearly observable in the recordings shown in FIGS. 3A, 3B, 3C and 3D.